The present invention relates to a voltage level shifter, and more particularly to a dual supply voltage input/out buffer capable of interfacing between two circuitry having different voltage swings.
Integrated circuits are becoming more dense as the dimensions of semiconductor devices decrease. The decreased dimensions also provide for faster devices that require less power to operate. In particular, semiconductor devices, such as transistors, operate on less voltage than the previous generation designs. At the beginning, 5 V power supply was commonly used. In the past few years, 3.3 V power supply has been replacing 5 V power supply as the dominant design. Recently, a move to 1.8 V power supply has been popularized among low power and high speed circuit designers.
One benefit of having lower operating voltage, and consequently lower power consumption, is that demands on the power supply is decreased. This is especially important where the portability of an electronic device incorporating these semiconductor devices is desired. Thus, applications may arise where both low and high voltage integrated circuits are connected together. An integrated circuit operating on a lower voltage must then provide an output at the higher voltage. Simply utilizing the high voltage in the low voltage integrated circuit is not practical. The structural integrity of the thin film oxide will suffer if exposed to voltage potentials in excess of the maximum rated supply, typically about 10% above the normal rated voltage. For example, a 3.7 V drop from the gate to the drain, source, or substrate will tend to break down the thin oxide manufactured by a 3.3 V process. A long term reliability issue will result, with the oxide breakdown eventually causing catastrophic damage to the transistor.
A specific concern of the above problems exist for 3.3 V drivers for 1.8 V process. Although various circuit design methods have been employed to allow 1.8 V devices to be built in 1.8 V processes that are tolerant of an incoming 3.3 V signal, true 3.3 V output drivers in a 1.8 process are sometimes required.